People from Bolt Beranek and Newman and others have extensively documented the Arpanet technology, including the Arpanet Interface Message Processor (IMP). This paper sketches the history (not the previously described technology) of the IMP program as originally written in 1969 for the modified Honeywell 516 computer. A sequence of other systems, evolving from the original software system and running on a variety of hardware platforms, are also enumerated. In 2013 a faded 1973 line printer listing of the IMP program was run through a special OCR program optimized to process such historical artifacts; an assembler was recreated to assemble the IMP code (looking like the modified PDP-1 Midas assembler used in 1973); and a software emulator of the original IMP hardware platform was created. This article also describes the methods used to recover a digital copy and assemble and run again the 1973 IMP code.